We propose to design, construct, and demonstrate an instrument based on magnetic markers and superconducting quantum interference device (SQUID) detectors for the goal of rapid and sensitive identification of 1-1000 biological particles in bioassays. Bioassays based upon magnetic tags would reduce or eliminate the reliance on radio-isotopes without compromising on the sensitivity of particle detection. Magnetic markers can also be detected in opaque media, such as blood or urine, since the signal is independent of the absorptive properties of complex media. Using SQUIDs, a significant reduction in the signal from non-specific binding might be achieved. Instruments for rapidly detecting and identifying a small number of individual biological particles without radioactivity and in complex samples could have broad applications in biomedical research, health care, occupational safety, and water monitoring. PROPOSED COMMERCIAL APPLICATION: SQUID-based instrument for rapidly identifying individual biological particles could have major applications in medicine, occupational safety, environmental protection, and national defense by reducing the potential battlefield and terrorist threats of biological warfare agents.